1. Field of the Invention
The present invention relates to an automatic focusing device adapted for use in a camera or the like.
2. Related Background Art
Most of the automatic focusing methods conventionally employed in the single-lens reflex cameras achieve focusing to an object by repeating a cycle of focus state detection (signal input from a sensor and calculation of focus state) and lens drive. The amount of lens movement in each cycle is based on the amount of defocus determined by the focus state detection in said cycle, and it is anticipated that the amount of defocus is brought to zero at the end of the lens drive.
The focus state detection and the lens movement naturally require a certain amount of time. However, in the case of a stationary object, the amount of defocus does not change unless the lens is moved, so that the amount of defocus to be cancelled at the end of lens movement is equal to the amount of defocus detected at the focus state detection, and correct focusing can therefore be achieved.
On the other hand, for a fast moving object, the amount of defocus varies in the course of focus state detection and lens movement, so that the amount of defocus to be cancelled may be significantly different from the detected amount of defocus. As a result, the focusing to the object may not be achieved at the end of lens movement.
Automatic focusing devices for avoiding such a drawback have been proposed in the Japanese Laid-Open Patents Sho 62-125311, Sho 62-139512, Sho 62-139511 and Sho 62-269936.
The automatic focusing devices disclosed in the above-mentioned patents are, in summary, to correct the amount of lens movement by foreseeing the change of the amount of defocus resulting from the movement of the object, in consideration of the change in the amount of defocus detected in said cycles and of the interval of said cycles, and are expected to improve the accuracy of focusing at the end of lens movement.
However, such an automatic focusing method involving correction may result in the following drawback when the object is stopped.
Even when the object is stopped, the result of focus state detection by detecting means is not constant but shows a certain fluctuation due, for example, to the influence of noises in said detecting means, and such fluctuating results may be misunderstood to indicate that the object is moving. Thus, the correction in such an automatic focusing method may lead to an improper focusing.
FIG. 9 is a chart showing the conventional method of correcting the amount of lens movement, indicating the position d of the image plane of the object in the ordinate, as a function of time in the abscissa.
A solid line f(t) indicates the image plane position of the object, while a broken line l(t) indicates the image plane position of the lens.
More detailedly, the line f(t) indicates the position, at time t, of the image plane of an object axially approaching to the camera when the focusing optical system of the photographing lens is focused at an infinite object distance, while the line l(t) indicates the position of the image plane of said object at the focusing state at time t of the focusing optical system. Each section [t.sub.i, t.sub.i '] indicates a focus state detecting operation, and each section [t.sub.i ', t.sub.i+1 ] indicates a lens driving operation.
Consequently, a so-called defocus amount is represented by the difference between f(t) and l(t) at the same time t, along the ordinate d. DFi is the defocus amount detected at a time t.sub.i ; DLi is the amount of lens movement determined from the focus state detection at a time t.sub.i-1 and represented by the change in the image plane position; and TMi is the interval in time of the focusing operations.
The conventional example shown in FIG. 9 is based on an assumption, for the corrective calculation, that the image plane position of the object varies according to a second-order function. More specifically, it is assumed, at a time t.sub.3, that the image plane position at a time t.sub.4 is foreseeable if three image plane positions (t.sub.1, f.sub.1), (t.sub.2, f.sub.2) and (t.sub.3 f.sub.3) at past and present are known.
In practice, however, the camera cannot detect the image plane positions f.sub.1, f.sub.2, f.sub.3 but the defocus amounts DF1, DF2, DF3 and the lens drive amounts DL1, DL2 represented in the amount of image plane movement. The future time t.sub.4 is unfixed and varies according to the change in the accumulating time of the charge accumulating sensor by the luminance of the object, but, for the purpose of simplicity in determining f.sub.4, t.sub.4 is defined as being known from t.sub.4 -t.sub.3 =t.sub.3 -t.sub.2.
Under the assumptions explained above, the lens drive amount in the section from t.sub.3 ' to t.sub.4, based on the result of the focus state detection at t.sub.3 is calculated according to the following relations: EQU a.t.sup.2 +b.t+c=f(t) (1) EQU a.t.sup.2.sub.1 +b.t.sub.1 +c=f(t.sub.1) (2) EQU a.t.sup.2.sub.2 +b.t.sub.2 +c=f(t.sub.2) (2') EQU a.t.sup.2.sub.3 +b.t.sub.3 +c=f(t.sub.3) (2")
Taking the point l.sub.1 in FIG. 9 as the original point; EQU f.sub.1 =DF1 (3) EQU f.sub.2 =DF2+DL1 (3') EQU f.sub.3 =DF3+DL2+DL1 (3") EQU t.sub.1 =0 (4) EQU t.sub.2 =TM1 (4') EQU t.sub.3 =TM1+TM2 (4")
The coefficients a, b and c are determined by substituting the equations (3), (3'), (3"), (4), (4'), and (4") into the equations (2), (2') and (2"): ##EQU1##
Consequently the lens drive amount represented in the amount of image plane movement DL3 at time t.sub.4 is given by: ##EQU2##
Thus DL3 is determined from the equation (8) on the aforementioned assumption that TM3=TM2.
Thereafter the lens drive amount at a time t.sub.i can be obtained in a similar manner, as indicated below, from the defocus amounts DFi-2, DFi-1, DFi in three past detections, the lens drive amounts DL1-2, DL1-1 in two past lens drives, and the two past time intervals TMi-2, TMi-1: ##EQU3##
Thus a proper focusing is obtained at the end of the lens driving operation even for a moving object, by determining the defocus amount DLi for the lens movement from the detected defocus DFi according to the equations (9), (10) and (11).
In the correcting method explained above, there are required data of at least two focusing operations in the past, in order to extrapolate the image plane position by a second-order function. However in the first two cycles of focusing, in which such data are not yet available, the lens is driven based on the detected defocus itself as shown in FIG. 9. In these cycles the correction is not applied by the correction means explained above. The actual corrective calculation is started from the third lens driving operation, and the effect of correction appears from the time t.sub.4, as shown in FIG. 9.
FIG. 10 shows a case of a stopped object, in which the lens is driven with a correction erroneously determined by misunderstanding the noises as a movement of the object. As in FIG. 9, the abscissa indicates time t, while the ordinate indicates the image plane position d of the object. However, a unit in the ordinate of FIG. 9 is in a magnified scale. A solid line f(t) indicates the image plane position of the object, while a folded line l(t) indicates the image plane position of the lens. Broken lines indicate the depth of focus of the optical system, by a value F .delta./2 on either side of the detected focus position, wherein F is the fully open F-number of the lens and .delta. is the size of a minimum aberration circle. Stated differently, photographing in a focused state is possible if the folded line l(t) is positioned inside an area defined by the broken lines.
In FIG. 10, the correction by the correction means is not applied from the cycle f.sub.1 to the defocus detection in f.sub.3. In the illustrated example, the lens is not driven in the cycle f.sub.1 since the defocus detected at (t.sub.1, f.sub.1) is within the depth of focus. However, the lens is driven at the next cycle because the defocus detected at (t.sub.2, f.sub.2) exceeds the depth of focus for some reason such as noise. At the next lens drive in response to the detection at (t.sub.3 f.sub.3), the abovementioned correction is added to obtain a result l.sub.4. However, even if the detection (t.sub.4, f.sub.4) provides a substantially correct result for the stopped object, a correction is added to the next lens drive to reach a result l.sub.5 by a misunderstanding that the object has moved. Consequently, said corrections bring the lens into defocus states at l.sub.4 and l.sub.5.